Shoulder muscle activation strategies differ when lifting or lowering a load

Type de document

Études primaires

Année de publication

2020

Langue

Anglais

Titre de la revue

European Journal of Applied Physiology

Première page

2417

Dernière page

2429

Résumé

Purpose: Lowering a load could be associated with abnormal shoulder and scapular motion. We tested the hypothesis that lowering a load involves different shoulder muscle coordination strategies compared to lifting a load.

Methods: EMG activity of 13 muscles was recorded in 30 healthy volunteers who lifted and lowered a 6, 12 or 18 kg box between three shelves. Kinematics, EMG levels and muscle synergies, extracted using non-negative matrix factorization, were analyzed.

Results: We found greater muscle activity level during lowering in four muscles (+ 1–2% MVC in anterior deltoid, biceps brachii, serratus anterior and pectoralis major). The movements were performed faster during lifting (18.2 vs. 15.9 cm/s) but with similar hand paths and segment kinematics. The number of synergies was the same in both tasks. Two synergies were identified in ~ 75% of subjects, and one synergy in the others. Synergy #1 mainly activated prime movers’ muscles, while synergy #2 co-activated several antagonist muscles. Synergies’ structure was similar between lifting and lowering (Pearson’s r ≈ 0.9 for synergy #1 and 0.7–08 for synergy #2). Synergy #2 was more activated during lowering and explained the greater activity observed in anterior deltoid, serratus anterior and pectoralis.

Conclusion: Lifting and lowering a load were associated with similar synergy structure. In 3/4 of subjects, lowering movements involved greater activation of a “multiple antagonists” synergy. The other subjects co-contracted all shoulder muscles as a unit in both conditions. These inter-individual differences should be investigated in the occurrence of shoulder musculoskeletal disorders. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.

Mots-clés

Épaule, Shoulder, Manutention manuelle, Manual handling, Soulèvement des charges, Manual lifting, Physiologie du muscle, Physiology of muscle

Numéro de projet IRSST

2014-0045

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